I received a number of complaints today about not being able to reach the mail server, DNS problems, not being able to retrieve web pages, etc. After some investigation I determined the problem was related to the communications server which failed yesterday. All machines on a LAN have an IP address and they have a physical hardware address (MAC address). They actually communicate over the LAN by the physical address. Each machine maintains a translation table (ARP table) that translates IP addresses into a physical address. They obtain this using a broadcast protocol initially then cache it in a table. This problem resulted from the fact that when the hardware was replaced, and thus had a new MAC address, other systems here still had the old ARP entry cached which tried to send IP traffic for the IP address of that replaced box to the old non-existant hardware address. I tried to manually delete these entries but SunOS is notorious about not wanting to let go of them so I had to reboot some machines to clear it out. Everything should be back to "normal" now.